This invention relates in general to electrostatographic imaging systems, and, in particular, to improved developer materials and their use.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is known as "cascade" development. In this method a developer material comprising relatively large carrier particles having finely divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the toner particles is so chosen as to have a triboelectric polarity opposite that of the carrier particles. As the mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier due apparently to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier particles and unused toner particles are then recycled. This technique is extremely good for the development of line copy images. The cascade development process is the most widely used commercial electrostatographic development technique. A general purpose office copying machine incorporating this technique is described in U.S. Pat. No. 3,099,943.
Another technique for developing electrostatic latent images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brushlike configuration. This "magnetic brush" is engaged with an electrostatic latent image bearing surface and the toner particles are drawn from the brush to the electrostatic latent image by electrostatic attraction.
Another technique for developing electrostatic latent images is the "touchdown" process as disclosed, for example, in U.S. Pat. Nos. 2,895,847 and 3,245,823 to Mayo. In this method a developer material is carried to a latent image bearing surface by a support layer such as a web or sheet and is deposited thereon in conformity with said image.
Carrier surfaces and carrier particles are generally made from or coated with materials having appropriate triboelectric properties as well as certain other physical characteristics. Thus, the materials employed as the carrier surfaces and carrier particles or the coatings thereon should have a triboelectric value commensurate with the triboelectric value of the toner to enable electrostatic adhesion of the toner to the carrier surface or carrier particles and subsequent transfer of the toner from the carrier surface or carrier particles to the image on the plate. Furthermore, the triboelectric properties of the carrier surface and all the carrier particles should be relatively uniform to permit uniform pickup and subsequent deposition of toner. The materials employed on the carrier surface and the carrier particles should preferably have an intermediate hardness so as not to scratch the plate or drum surface upon which the electrostatic image is initially placed while being sufficiently hard to withstand the forces to which they are subjected during recycle. The carrier substrate and carrier particles as well as the surface thereof also should not be comprised of materials which are so brittle as to cause either flaking of the surface or particle breakup under the forces exerted on the carrier during recycle. The flaking thereof causes undesirable effects in that the relatively small flaked particles may eventually be transferred to the copy surface thereby interfering with the deposited toner and causing imperfections in the copy image. Furthermore, flaking of the carrier surface will cause the resultant carrier to have nonuniform triboelectric properties when the carrier is composed of a material different from the surface coating thereon. This results in undesirable nonuniform pickup of toner by the carrier and nonuniform deposition of toner on the image. In addition, when the carrier particle size is reduced, the removal of the resultant small particles from the plate becomes increasingly difficult. Thus, the types of materials useful for making carrier or for coating carrier, although having the appropriate triboelectric properties, are limited because other physical properties which they possess may cause the undesirable results discussed above.
While ordinarily capable of producing good quality images, conventional developing materials suffer serious deficiencies in certain areas. The developing materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the electrostatographic process. Some developer materials, though possessing desirable properties such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge and agglomerate during handling and storage. Adherence of carrier particles to reusable electrostatographic imaging surfaces causes the formation of undesirable scratches on the surfaces during the image transfer and surface cleaning steps. The tendency of carrier particles to adhere to imaging surfaces is aggravated when the carrier surfaces are rough and irregular. The coatings of most carrier particles deteriorate rapidly when employed in continuous processes which require the recycling of carrier particles by bucket conveyors partially submerged in the developer supply such as disclosed in U.S. Pat. No. 3,099,943. Deterioration occurs when portions of or the entire coating separates from the carrier core. The separation may be in the form of chips, flakes or entire layers and is primarily caused by fragile, poorly adhering coating material which fails upon impact and abrasive contact with machine parts and other carrier particles. Carriers having coatings which tend to chip and otherwise separate from the carrier core or substrate must be frequently replaced thereby increasing expense and loss of productive time. Thus, generally, coated carrier particles having coatings which tend to chip or separate from the carrier core cannot be reclaimed and reused after many machine cycles. Print deletion and poor print quality occur when carriers having damaged coatings are not replaced. Fines and grit formed from carrier disintegration tend to drift and form undesirable and damaging deposits on critical machine parts. Many carrier coatings having high compressive and tensile strength either do not adhere well to the carrier core or do not possess the desired triboelectric characteristics. In addition, carriers having discontinuous coatings generally promote adhesion failure between the carrier substrate and the carrier coating material giving rise to the aforementioned problems and result in variations in triboelectric characteristics, premature discharge of the photoconductive imaging surface causing degradation of the electrostatic latent image, scratching of the imaging surface, not to mention manufacturing difficulties in reproducing carriers having discontinuous coatings. Further the triboelectric and flow characteristics of many carriers are adversely affected when relative humidity is high. For example, the triboelectric values of some carrier coatings fluctuate with changes in relative humidity and are not desirable for employment in electrostatographic systems, particularly in automatic machines which require carriers having stable and predictable triboelectric values.
Another factor affecting the stability of carrier triboelectric properties is the susceptibility of carrier coatings to "toner impaction." When carrier particles are employed in automatic machines and recycled through many cycles, the many collisions which occur between the carrier particles and other surfaces in the machine cause the toner particles carried on the surface of the carrier particles to be welded or otherwise forced onto the carrier surfaces. The gradual accumulation of impacted toner material on the surface of the carrier causes a change in the triboelectric value of the carrier and directly contributes to the degradation of copy quality by eventual destruction of the toner carrying capacity of the carrier.
It has been ascertained that in order to develop a latent image comprised of negative electrostatic charges, an electroscopic powder and carrier combination should be selected in which the powder is triboelectrically positive to the granular carrier; and to develop a latent image comprised of positive electrostatic charges, an electroscopic powder and carrier should be selected in which the powder is triboelectrically negative to the carrier. It is often desirable in any type of printing to produce a reverse copy of an original. By this is meant to produce a negative copy from a positive original or, on the other hand, a positive copy from a negative original. In electrostatographic printing, image reversal can be accomplished by applying to the image a developer powder which is repelled by the charged areas of the image and adheres to the discharged areas.
The triboelectric relationship between the electroscopic powder and the carrier depends on their relative positions in a triboelectric series in which the materials are arranged in such a way that each material is electrostatically charged with a positive charge when contacted with any material below it in the series and with a negative charge when contacted with any material above it in the series. In the reproduction of high contrast copy such as letters, tracings, etc., it is desirable to select the electroscopic powder and carrier materials so that their mutual electrification is sufficient to cause the toner particles to electrostatically cling to the carrier surface, and the degree of such electrification is normally governed by the distance between their positions in the triboelectric series, that is, the greater distance they are removed from one another, the greater the mutual electrification and the closer they are together in the series, the less the mutual electrification.
It is highly desirable to control the triboelectric properties of carrier surfaces to accommodate the use of desirable toner compositions while retaining the other desirable physical characteristics of the carrier. The alteration of the triboelectric properties of a carrier by applying a surface coating thereon is a particularly desirable technique. With this technique, not only is it possible to control the triboelectric properties of a carrier made from materials having desirable physical characteristics, it is also possible to employ materials previously not suitable as a carrier. Thus, for example, a carrier having desirable physical properties with the exception of hardness, can be coated with a material having desirable hardness as well as other physical properties, rendering the resultant product more useful as a carrier.
Suitable coated and uncoated carrier materials for cascade, magnetic brush, and touchdown development are well known in the art. The carrier comprises any suitable solid material, provided that the carrier acquires charge having an opposite polarity to that of the toner particles when brought in close contact with the toner particles so that the toner particles adhere to and surround the carrier. By proper selection of material in accordance with their position in the triboelectric series, the polarities of their charge when the materials are mixed are such that the electroscopic toner particles adhere to and are coated on the surface of a carrier and also adhere to that portion of the electrostatic image bearing surface having a greater attraction for the toner than the carrier.
For a carrier coating material to be useful in preparing carriers for reversal development, it should have the proper triboelectric properties. A vinyl chloride-vinyl acetate copolymer, as disclosed by L. E. Walkup in U.S. Pat. No. 2,618,551, is used for coating a carrier for use in reversal development of positively charged images. However, this copolymer is not spaced far enough below many toner materials in the triboelectric series to provide high quality reversal images. Therefore, a dye is used to enhance the reversal character of the carrier coating. While the carrier prepared from this polymer-dye blend has utility, it also has disadvantages. Batch to batch uniformity is poor. High speed machines requiring high quality output have great difficulty when trying to use this carrier. The origin of these difficulties probably lies in the incomplete compatibility of the dye with the polymer and possibly may be due to leaching of the dye from the carrier coating composition.
Thus, there is a continuing need for a better developer material for developing electrostatic latent images.